Session TO4: EAST

Overview of long pulse H-mode operation on EAST

The EAST research program aims to demonstrate steady-state long-pulse high-performance H-mode operations with ITER-like poloidal configuration and RF-dominated heating schemes. In the recent experimental campaign, a long pulse fully non-inductive H-mode discharge lasting over 100 seconds using the upper ITER-like tungsten divertor has been achieved in EAST. This scenario used only RF heating and current drive, but also benefitted from an integrated control of the wall conditioning, plasma configuration, divertor heat flux, particle exhaust, impurity management and superconducting coils safety. Maintaining effective coupling of multiple RF heating and current drive sources on EAST is a critical ingredient. This long pulse discharge had good energy confinement, H98,y2 \textasciitilde 1.1-1.2, and all of the plasma parameters reach a true steady-state. Power balance indicates that the confinement improvement is due partly to a significantly reduced core electron transport inside minor radius rho\textless 0.4.   

RF heating and current drive with dominant electron Heating in Long Pulse Discharges on EAST

The efficient heating and current drive (H{\&}CD) with dominant electron heating capability using Radio Frequency (RF) Continuous Wave (CW) actuators (LHW, ICRF, ECRH) has been successfully demonstrated with ITER-like low torque conditions in recent EAST fully non-inductive long-pulse H-mode discharges. The plasma was sustained by \textasciitilde 70{\%} RF H{\&}CD together with \textasciitilde 30{\%} bootstrap current fraction. A high electron temperature plasma (core Te\textgreater 4.0keV) was obtained, resulting in lower reduced loop voltage, through the investigation possibly due to the synergisticy effect between ECRH and LHCD. In separate experiments, the characteristic effect of LH wave frequency (a comparison of 2.45 GHz and 4.6 GHz LHW) on LHCD showed that the higher LH frequency is preferred favorable for current drive at high density. The improvement in LHCD at higher frequency is mainly ascribed to a reduction in the Parametric Instability (PI) and to a lesser extent, Collisional Absorption (CA) in the edge region. Finally, new recent experiments shows that the higher frequency LH improves penetration of the coupled RF power into the plasma core, also leading to a better effect on plasma characteristics more efficient core heating. The detailed results will be given.   

Particle and Power Exhaust in EAST

A total power injection up to 0.3GJ has been achieved in EAST long pulse USN operation with ITER-like water-cooling W-monoblock divertor, which has steady-state power exhaust capability of 10 MWm$^{\mathrm{-2}}$. The peak temperature of W target saturated at t $=$ 12 s to the value T$\approx $500$^{\mathrm{o}}$C and a heat flux $\approx $3MWm$^{\mathrm{-2\thinspace }}$was maintained. Great efforts to reduce heat flux and accommodate particle exhaust simultaneously have been made towards long pulse of 10$^{\mathrm{2}}$s time scale. By exploiting the observation of Pfirsch--Schlüter flow direction in the SOL, the Bt direction with Bx$\nabla $B away from the W divertor (more particles favor outer target in USN) was adopted along with optimizing the strike point location near the pumping slot, to facilitate particle and impurity exhaust with the top cryo-pump. By tailoring the 3D divertor footprint through edge magnetic topology change, the heat load was dispersed widely and thus peak heat flux and W sputtering was well controlled. Active feedback control of total radiative power with neon seeding was achieved within $f_{rad}=$17-35{\%}, exhibiting further potential for heat flux reduction with divertor and edge radiation. Other heat flux handling techniques, including quasi snowflake configuration, will also be presented.   

Integrated Plasma Control for Alternative Plasma Shape on EAST

To support long pulse plasma operation in high performance, a set of plasma control algorithms such as PEFIT real-time equilibrium reconstruction, radiation feedback, Beta and loop voltage feedback and quasi-snowflake shape f control have been implemented on EAST Plasma Control system (PCS) which was adapted from DIII-D PCS. PEFIT is a parallelized version of EFIT by using GPU with highest computation acceleration ratio up to 100 with respect to EFIT. It demonstrated high performance both in DIII-D data analysis and in the real-time shape control on EAST plasma either in normal or quasi-snowflake shape. Loop voltage has been successfully controlled by Low Hybrid Wave (LHW) while the plasma current is maintained by poloidal field coil set. Beta control has been also demonstrated by using LHW and it will be extended to other heating sources because the PCS interface is ready. Radiation feedback control has been achieved by Neon seeding by Super-Sonic Molecular Beam Injection (SMBI). For the plasma operation in quasi-snowflake, we have reached 20 s ELMy free high confinement non-inductive discharges with betap \textasciitilde 2, H98 \textasciitilde 1.1 and plasma current \textasciitilde 250 kA.   

Multimode Plasma Response in the EAST Tokamak.

A multimodal plasma response to n $=$ 2 RMPs was found in EAST tokamak low $\beta_{N} $ plasmas. The signature of the multimodal response is the difference in the radial ($B_{r} )$ and poloidal ($B_{p} )$ magnetic field dependencies on the applied phasing (poloidal structure). A difference in the 3D coil phasing that maximizes these two responses is observed only in response to n$=$2 fields, while the n$=$1 $B_{r} $ and $B_{p} $ have the same phasing dependence. Neither the maximum $B_{r} $ nor $B_{p} $ agrees with the best phasing for ELM mitigation in experiments. GPEC modeling accurately reproduces the experimental measurements only when multiple eigenmodes of the plasma response are included. The measured plasma response is not dominated by resonant current drive from the external field, with non-resonant contributions playing a large role. These results on EAST demonstrate a new type of multimode response based on the variation of the polarization of the plasma response to the 3D field. The results clearly demonstrate the danger of associating any one sensor array with desired physics consequence of 3D fields and the need for 3D modeling to predict optimal 3D field configurations in multi-modal plasmas.   

Vertical Position and Current Profile Measurements by Faraday-effect Polarimetry On EAST tokamak

A primary goal for ITER and prospective fusion power reactors is to achieve controlled long-pulse/steady-state burning plasmas. For elongated divertor plasmas, both the vertical position and current profile have to be precisely controlled to optimize performance and prevent disruptions. An eleven-channel laser-based \textbf{PO}larimeter-\textbf{INT}erferometer (POINT) system has been developed for measuring the internal magnetic field in the EAST tokamak and can be used to obtain the plasma current profile and vertical position. Current profiles are determined from equilibrium reconstruction including internal magnetic field measurements as internal constraints. Horizontally-viewing chords at/near the mid-plane allow us to determine plasma vertical position non-inductively with subcentimeter spatial resolution and time response up to 1 \textunderscore s. The polarimeter-based position measurement, which does not require equilibrium reconstruction, is benchmarked against conventional flux loop measurements and can be exploited for feedback control. Work supported by US DOE through grants \textit{DE-FG02-01ER54615 and }DC-SC0010469.   

ELM elimination with lithium aerosol injection in upper-single null discharges using the tungsten divertor in EAST

A reproducible, fully non-inductive H-mode regime devoid of large ELMs has been achieved by continuous Li injection in EAST into the upper `ITER-like' tungsten divertor, extending previous results on the graphite divertor [J.S. Hu et al., PRL 114 (2015) 055001]. These discharges did not suffer from density or impurity accumulation, and maintained constant core radiated power. The new results extend the energy confinement multiplier H98(y,2) \textasciitilde 1.2, as compared to H98(y,2) \textasciitilde 0.75 previously on the graphite divertor. The observed ELM elimination is correlated with a decrease in particle recycling, as expected from the strong Li coating before the experiment, and real-time Li aerosol injection. In addition, core W concentration was reduced during the Li injection. ELM elimination is likely related to the reduced recycling and density /temperature profile changes. A low-n electromagnetic coherent mode (MCM) at \textasciitilde 40kHz became stronger in amplitude and also more coherent. The MCM shows strong magnetic fluctuations as measured by fast Mirnov coils, but weak density fluctuations. As compared to the graphite divertor, Li injection into the tungsten divertor eliminated ELMs at twice the previous auxiliary heating power, and reduced pedestal collisionality.   

Progressive recycling reduction with Li aerosol injection in EAST

Recent experiments at the EAST tokamak have shown a strong reduction in divertor recycling when lithium is injected into the plasma, with tungsten used as the wall material in the active divertor. These extend previous studies using carbon as the wall material onto which lithium is applied, and test the ability to use lithium aerosol injection for active wall conditioning during long-pulse plasmas. Reduced recycling, as evidenced by D? emission, is observed during the lithium injection, qualitatively like that observed in other experiments using pre-discharge lithium deposition. The magnitude of the reduction increases with the lithium injection rate, with up to a factor of \textasciitilde 2 observed. The recycling reduction is most pronounced in the active divertor, consistent with the aerosol being transport by the plasma preferentially to the strongly plasma-wetted regions. In contrast, ion flux is affected more weakly, decreasing by less than 20{\%} under lithium injection. Modeling with the SOLPS plasma/neutrals transport code indicates a relative reduction in the divertor recycling coefficient of \textasciitilde 20{\%} (e.g., R$=$0.99 to 0.8) with lithium injection. These results show the potential for lithium injection to provide real-time control of recycling and particle removal via surface pumping. *US scientists supported by U.S. DOE Contracts DE-AC05-00OR22725, DE-AC02-09CH11466, DE-FC02-04ER54698, and ASIPP scientists by Contract Nos. 2017YFA0402500, 11625524, 11075185, 11021565, 2013GB114004.   

ELM Suppression and performance improvement with a flowing liquid lithium limiter in EAST

Improvements in plasma performance were observed using a second-generation flowing liquid lithium limiter (FLiLi) in EAST. Compared to the H mode discharges without FLiLi, ELM frequency and amplitude were both lower with FLiLi. Also, ELM frequency and amplitude gradually decreased discharge-by-discharge with FLiLi, similar to the gradual ELM mitigation by real-time Li aerosol injection in successive discharges. Moreover, transient ELM-free H-modes with a strong increase of W$_{\mathrm{MHD}}$ and H$_{\mathrm{98}}$ were observed for the first time with FLiLi. During the ELM-free phases, MHD activity interpreted from high frequency Mirnov probes differed from activity in the ELMy phases. In addition to the typical low-frequency \textasciitilde 50 kHz edge coherent MHD mode (ECM), a second mode \textasciitilde 220--240 kHz also was observed in the ELM-free phase. By computing the Li efflux from the FLiLi limiter surface, it was found that the Li efflux from sputtering during discharges and evaporation between discharges was comparable to the typical mass delivery rates used for Li powder injection rate during plasma operation in EAST. Therefore, gradual accumulation of Li in EAST via real time Li efflux from the FLiLi surface produces similar effects to aerosol injection, i.e. reduced recycling, enhanced fluctuations, and ELM mitigation.   

Tungsten control in long pulse H-mode discharges on EAST

Tungsten impurity is well controlled in EAST H-mode discharges applying on-axis RF heating and Li wall conditioning, which provide great benefit to achieve the reproductive long pulse H-mode discharges (pulse length \textgreater 60s) with low level of tungsten concentration (Cw), e.g. 3.0x10$^{\mathrm{-6}}$-1.5x10$^{\mathrm{-5}}$. It was found that the tungsten accumulation can be suppressed by increasing ELM frequency after superimposing 4.6GHz LHW on the NBI phase [L. Zhang et al., NME 2017]. On-axis ECRH with power of 0.35MW is also very effective to control tungsten in core plasma. After injection of ECRH, Cw decreases up to 40{\%} and the peaked profile of tungsten in the core plasma becomes hollow, suggesting a weakened neoclassical tungsten transport in the core region. The real-time Li aerosol injection has also been applied to enhance the particle recycling control on EAST. It is found that with Li aerosol injection, stable profiles of tungsten are sustained both in the plasma core and at the edge, while the concentrations are halved compared to the normal H-mode discharge, suggesting a reduced tungsten source. The mechanism of effective tungsten control will be further discussed in this work.   

Prompt triggering of edge localized modes through lithium granule injection on EAST

We report successful triggering of edge localized mode (ELMs) in EAST with Lithium (Li) micropellets, and the observed dependence of ELM triggering efficiency on granule size. ELM control is essential for successful ITER operation throughout the entire campaign, relying on magnetic perturbations for ELM suppression and ELM frequency enhancement via pellet injection. To separate the task of fueling from ELM pacing, we initiate the prompt generation of ELMs via impurity granule injection. Lithium granules ranging in size from 200 - 1000 microns are mechanically injected into upper-single null EAST long pulse H-mode discharges. The injections are monitored for their effect on high Z impurity accumulation and to assess the pressure perturbation required for reliable ELM triggering. We have determined that granules of diameter larger than 600 microns (corresponding to 5.2 x 10$^{\mathrm{18}}$ Li atoms) are successful at triggering ELMs more than 90{\%} of the time. The triggering efficiency drops precipitously to less than 40{\%} as the granule size is reduced to 400 microns (1.5 x 10$^{\mathrm{18}}$ Li atoms), indicating a triggering threshold has been crossed. Using this information an optimal impurity granule size which will regularly trigger a prompt ELM in these EAST discharges is determined. Coupling these results with alternate discharge scenarios on EAST and similar experiments performed on DIII-D provides the possibility of extrapolation to future devices.   

Investigation of the harmonic coherent modes in the EAST pedestal region

A coherent mode structure, with up to 7 harmonics of a fundamental frequency of 12-15 kHz , has been observed in pedestal region during EAST H-mode phase driven by lower hybrid current drive (LHCD). Although such harmonic coherent mode (HCM) has a few similar features as the edge harmonic oscillation (EHO) in the DIII-D QH-mode, some differences between these two modes can still be discerned. The HCM peaks in the steep gradient region of pedestal (near the pedestal top), and has the toroidal mode numbers ranging from n $=$ 1 to 7. The edge radial electric field well during the H-mode phase with HCM is much deeper than that without ELM. Moreover, the appearance of HCM is always accompanied by the suddenly decreased Dalpha signals, and the amplitude of fundamental branch is usually smaller than that of higher-order ones, which could be roughly explained by the free energy redistributed among all the harmonic branches through the amplitude correlation method. Bi-spectral analysis indicates that strong coupling between HCM and high frequency turbulence exists at the HCM peaking location, implying the important role of turbulence in HCM saturation process.   

New Small-ELM H-mode Regimes for Steady-state High-performance Operations in EAST

A stationary high-confinement (H$_{\mathrm{98y2}}$\textasciitilde 1.1) fully non-inductive H-mode regime characterized by high-frequency (2kHz) small ELMs (divertor peak heat flux \textasciitilde 2MW/m$^{\mathrm{2}})$ at relatively low pedestal collisionality ($\nu $*$_{\mathrm{e}}$\textasciitilde 1) and optimized high internal inductance (l$_{\mathrm{i}}$\textasciitilde 1.1) plasma has been recently achieved with high heating power (source power \textasciitilde 9MW) in EAST. This regime was obtained at high triangularity $\delta $ (0.55), high q$_{\mathrm{95}}$ (6) and high $\beta_{\mathrm{p}}$ (1.6), close to the parameter space of the grassy ELM regime in JT-60U. The relatively low plasma current, high q$_{\mathrm{95}}$, $\beta_{\mathrm{p}}$, and therefore high bootstrap current fraction (\textgreater 30{\%}) make it a suitable regime to achieve steady-state operation with low disruptivity, good reproducibility and robustness. The access to this regime is insensitive to the change of toroidal torque or plasma density in the explored parameter range. Benefiting from the optimized high l$_{\mathrm{i}}$ and high $\beta _{\mathrm{p}}$, good core energy confinement has been achieved even at low core toroidal rotation ($V_{t}$ down to 10 km/s). Good density control has been achieved at a line-averaged density up to 76{\%} Greenwald. Impurity concentration and core radiation were maintained at an acceptably low level, suggesting that sufficient particle exhaust can be driven by the high-frequency small ELMs.   

Diagnostic and Hardware Upgrades for the US-PRC PMI Collaboration on EAST

Several collaborative diagnostic and hardware upgrades are planned to improve understanding and control of Plasma-Material Interactions on EAST, as part of the US-PRC PMI collaboration. Dual-band thermography adapters, designed by UT-K and ORNL, are being designed for existing IR cameras to improve the accuracy of the divertor heat flux measurements by reducing sensitivity to surface emissivity. These measurements should improve power accounting for EAST discharges, which can show a large gap between input power and divertor exhaust power. MIT is preparing tungsten tiles with fluorine depth markers to measure net erosion of PFC tiles. JHU plans to improve the electronics of the Multi-Energy Soft X-ray diagnostic as well as expand the present edge system to a full core-edge measurement; this will enhance the assessment of the effect of Li injection on tungsten accumulation and transport. In addition to PPPL-developed upgrades to the lithium granule and pellet delivery systems, LANL is assessing core-shell micropellets for pellet ablation analysis. Finally, UIUC and PPPL are developing flowing liquid lithium limiters, both with and without LiMIT tile features, for deployment on EAST.   

Edge simulations in ELMy H-mode discharges of EAST tokamak

Simulations of ELM crash followed by a coherent mode, leading to transient divertor heat flux on EAST are achieved by the six-field two-fluid model in BOUT$++$. Three EAST ELMy H-mode discharges with different pedestal structure, geometry and plasma current Ip are studied. The ELM-driven crash of the profiles in pedestal is reproduced, and the footprints of ELM filaments on targets are comparable with the measurements from divertor probes. A coherent mode is also found in the edge region in all the simulations after the ELM crash. The frequency and poloidal wave number are in the range of the edge coherent mode (ECM) on EAST. The magnetic fluctuations of the mode are smaller than the electric field fluctuations. The detailed comparisons between simulated mode structures with measurements will be reported. Statistical analysis on the simulated turbulent fluctuations shows that both the turbulent and blobby electron anomalous transport can pump the pedestal energy out into SOL, and then flow to divertors. The similar trend of the heat flux width with Ip is obtained in the simulations. The effects of the SOL current driven by LHW on ELMs will be discussed in this paper.